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Ash free dry biomass
Why is this necessary?
Both the water content and gut contents of an earthworm greatly affects its fresh weight (the same earthworm with and empty gut will weight a lot less than the same earthworm with a full gut). When it comes to getting a measure that can reliably compared to other measurements in other places and at other times, this is important.
To eliminate the variability due to water content you can simply dry the earthworm at about 60° C for 24-48 hours to get the “dry weight” (scientists use this same technique to get reliable weights of plants and soils too). But removing the gut contents poses a unique problem. To eliminate the variability in weight due to gut contents, first we weight the dry earthworm, then we ash the earthworm (burn it in a special oven at 500° C for several hours) which burns off all of the organic parts of the earthworm leaving only the mineral soil that was in the earthworm gut. Then we weight the ash and subtract that amount from the earthworm dry weight to get “ash-free dry weight”. Here is an example of how these different things can affect your measurement of earthworm biomass:
1.21 grams (fresh weight of a nightcrawler (Lumbricus terrestris))
- 0.24 grams (water content of the nightcrawler)
= 0.97 grams (dry weight of the nightcrawler)
0.97 grams (dry weight of the nightcrawler)
-0.29 grams (gut contents (ash) of the nightcrawler)
= 0.68 grams (ash-free dry biomass of the nightcrawler)
To see how much of a difference moisture and gut contents can make to your biomass measure; you can do this study which uses the same “loss by ignition” method (a fancy way of saying you burn up the carbon) described for determining soil organic matter content. If you have access to a muffle oven that can reach 500°C, then try this process, you may be surprised by what you find!
The process
- Weigh your fresh earthworm (preserved or unpreserved), to the nearest 0.0001 grams.
- Dry your earthworm(s) in a drying oven at 60° C for 24-48 hours (or until it no longer loses weight) to drive off all remaining soil water content.
- Weight your fully dry earthworm sample(s) to the nearest 0.0001 grams.
- Place your earthworm in a muffle oven at 500° C for a minimum of 4 hours, to burn off all the combustible parts of the earthworm, basically everything except the gut contents.
- Weigh the ash, what remains of the burned earthworm(s), to the nearest 0.0001 grams.
- Subtract the ash weight from the dry weight of your earthworm to get ash-free-dry biomass of your earthworm.
If you want to do this kind of a study using individual earthworms, use large individuals like Lumbricus terrestris, since individuals of smaller species often don’t weight very much by the end! Otherwise, you could use multiple individuals and get an average for the group. Be sure to use a scale that is accurate to .0001 grams!